Television and like receiver



y 1940. T. c. NUTTALL 2,200.009

TELEVISION AND LIKE RECEIVER Filed April 17, 1959 K W 1 Jung m 0 i J 11T T Ipmw rol Patented May 7, 1940 UNITED STATES PATENT OFFICE TELEVISIONAND LIKE RECEIVER Application April 17, 1939, Serial No. 268,397

- In Great Britain April 29, 1938 8 Claims.

This invention relates to improvements in television and like receiversand is especially concerned with circuits for separating line frequencyand frame frequency synchronising impulses which are transmitted asexcursions of a carrier in the same sense from a datum level.Synchronising impulses employed in television systems commonly comprisetwo series of substantially rectangular impulses of differentperiodicity which are transmitted as variations of carrier amplitude ina range outside that occupied by picture signals, the two sets ofimpulses being of the same amplitude but having different time lengths.It is advantageous to separate the impulses of lower periodicity fromthose of higher periodicity before applying them to control anoscillation generator which is required to be operative at the lowerperiodicity in order to prevent incorrect operation of the saidoscillation generator, especially in systems in which intercalatedscanning is employed.

The present invention seeks to provide an improved method of effectingthis separation.

According to the present invention there is provided a method ofseparating synchronising impulses of a greater duration fromsynchronising impulses of a lesser duration which comprises applyingoriginal impulses to a network such as to produce a time delay at leastequal to the time length of the shorter impulses, mixing the delayedimpulses with the original impulses and applying the combined impulsesto a device adapted to pass only signals exceeding the individualamplitude of the delayed and original impulses when mixed.

According to a feature of the present invention synchronising impulsesare applied to the input terminals of a network of which the output isunterminated so that reflection occurs with the result that signalsapplied to the input terminals of the network reappear at theseterminals after a time equal to twice the delay time of the network.

In order that the present invention may be more particularly describedreference is now made to the accompanying diagrammatic drawing, ofwhich:

Figure 1 is a diagram illustrating the method of operation of thepresent invention;

' Figures 2 and 3 are block schematic diagrams illustrating differentmethods of carrying out the present invention, and

Figure 4 shows the circuit of a delay network suitable for carrying outthe present invention in one mode.

In Figure 1, A represents the waveform of synchronising impulsesreceived from a transmitter, the higher frequency impulses beingrepresented by l and a lower frequency impulse by 2. B represents thesame signals delayed by a time greater 5 than the time length of thehigher frequency impulses but less than the time length of the lowerfrequency impulses, while C represents the combination of original anddelayed impulses. It will be seen that in C the lower frequency im- 10pulses have been increased in magnitude so that by applying the impulsesto a device passing only signals exceeding a value indicated by the line3, effective separation of low frequency from high frequency impulses isobtained.

In Figure 2 mixed high frequency and low frequency synchronisingimpulses are applied to the terminals 4 and pass on by two differentchannels to a mixing device I and thence to a limiting device 5 passingonly signals above a chosen magnitude. One of the two channels comprisesa delay network 6 while the other introduces no delay. If the delayintroduced by the network 6 is greater than the duration of the higherfrequency impulses the effect described in relation to Figure 1 willoccur and, by suitable adjustment of the cut-off point of the device 5,low frequency synchronising impulses only will appear at the terminals8.

In Figure 3 use is made of the fact that reflection of signals willoccur from the open end of an unterminated network. Signals are appliedat the terminals 4 through a resistance 9 to a delay network 6, theoutput terminals III of which are unterminated. Thus at the terminals llsignals will appear comprising both applied and reflected impulses andif, as before, the delay period of the network 6 and the cut-oil? pointof the limiting device 5 are suitably chosen, low frequency impulsesonly will appear at the terminals 8.

As described, the invention calls for the use of a non-dissipativenetwork designed to simulate a length of line having input and outputterminals. However, the output terminals of this network are opencircuited and are not required for connection to any other part of thecircuit, only the two input terminals being utilised; I may thereforecarry out the invention by the use of any two-terminal network whichproduces appropriate reactance variations. This is an advantage in sofar as it allows greater freedom in the design of the network.

' The nature of the required reactance variations with frequency may bedetermined by comparison with the case of an ideal open-circuited line."As is well known, the reactance in this case follows a cotangent law,points of infinite and zero reactance occurring at equidistant-frequencyintervals. It is not possible to reproduce this law exactly with afinite number of circuit elements, and the greater the accuracy requiredthe greater the number of elements which must be used. I have found,however, that for many purposes the simplest form of circuit shown inFigure 4 is suflic-iently accurate. This circuit, which is fed fromterminals 4 through resistance 9 as before, comprises a seriesinductance l2 shunted at either end by condensers l3 and M respectively.For separating low frequency impulses of approximately 40 micro-secondsduration from high frequency impulses of approximately 10 microsec--onds duration the following values were found satisfactory in practice:

Resistance 9 ohms 7850 Inductance l2 millihenries 20 Condenser I3microfarad 0.0001

Condenser i l do 0.0005

I claim:

1. A method of separating synchronising impulses of a greater durationfrom synchronising impulses of a lesser duration which comprisesapplying original impulses to a network such as to produce a time delayat least equal to the time length of the shorter impulses, mixing thedelayed impulses with the original impulses and applying the combinedimpulses to a device adapted to pass only signals exceeding theindividual amplitude of the delayed and original impulses when mixed.

2. A method of separating relatively long and short electric impulseswhich comprises producing a time delay of the impulses for an intervalat least equal to the length of the short impulses, but less than thelength of the long impulses, mixing the delayed impulses with theoriginal impulses, whereby a difference in amplitude is obtained for thelong and short impulses, and separating the long and short impulses byselection according to the difference in amplitude.

3. A method of separating relatively long and short electric impulseswhich comprises producing a time delay of the impulses for an intervalat least equal to the length of the short impulses but less than thelength of the long impulses, adding the delayed impulses to theoriginally applied impulses, whereby only the long impulses acquire alarger amplitude than the originally applied impulses, and selectingonly the impulses exceeding the individual amplitude of the delayed andoriginally applied impulses.

4. A method of separating relatively long and short electric impulseswhich comprises applying the impulses to an open circuited artificialtransmission line to produce reflected impulses delayed by at least thelength of the short impulses but less than the length of the longimpulses, whereby Y the reflected impulses are added to the originallyapplied impulses and selecting from the combined originally applied andreflected impulses producing a delay at least equal to the duration,

of the short impulses but less than the duration of the long impulses, amixing device for the originally applied impulses and the delayed impulses, whereby an increase in amplitude of the long impulses isobtained, and an amplitude selecting device for the combined impulsesfrom the mixing device.

6. Apparatus for separating relatively long and short electric impulsescomprising a two-terminal time delay network producing a delay which isat least one-half the duration of the short impulses, but less thanone-half the duration of the long impulses, a mixing device, meansapplying the impulses to said mixing device and to said network, wherebythe originally applied impulses are combined with reflected impulsesfrom said network with a resulting increase in the amplitude of the longimpulses and an amplitude selecting device for the combined impulsesfrom the mixing device.

7. Apparatus for separating relatively long and short electric impulsesas claimed in claim 6, wherein the two-terminal time delay network has areactance-frequency characteristic approximating to that of an idealopen circuited line.

8. Apparatus for separating relatively long and short electric impulsesas claimed in claim 6, wherein the two-terminal time delay networkcomprises at least one inductance and two condensers producing a zeroreactance at one frequency and infinite reactance at another frequency.

THOMAS CAYTON NUTTALL.

